Ongoing studies of parvovirus have shown that long passage of integrated AAV carrier cell clones may show small amounts of extrachromosomal viral DNA. Studies of the polypeptides of "human" paroviruses demonstrate that these viruses are structurally and immunologically closely related to rodent (RV) parvovirus and that these viruses undergo a post assembly maturation process involving protolytic cleavage. It has been further shown that the immunogenic expression of a cross reacting antigen between large plaque rat virus (LPRV) and minute virus of mice is linked to the host cells in which these viruses are grown. Continued studies of AAV proteins have shown that each of the three recognized structural polypeptides are closely related and show a great amount of protein sequence homology. One unexpected finding is that all of the polypeptides of canine AAV (CAAV) appear 10-20% larger than those of all other AAV's thus far studied. We have some evidence that suggests this is the result of a block in CAAV virion maturation.